Dispersed oxide reinforced martensitic steel excellent in high temperature strength and method for production thereof

ABSTRACT

In an oxide dispersion strengthened martensitic steel which comprises, by % by weight, 0.05 to 0.25% C, 8.0 to 12.0% Cr, 0.1 to 4.0% W, 0.1 to 1.0% Ti, 0.1 to 0.5% Y 2 O 3  with the balance being Fe and unavoidable impurities and in which Y 2 O 3  particles are dispersed in the steel, by adjusting the Ti content within the range of 0.1 to 1.0% so that an excess oxygen content Ex.O in steel satisfies [0.22×Ti (% by weight)&lt;Ex.O (% by weight)&lt;0.46×Ti (% by weight)], the oxide particles are finely dispersed and highly densified to thereby obtain an oxide dispersion strengthened martensitic steel excellent in high-temperature strength. It is also possible to reduce the amount of oxygen contamination in steel during the mechanical alloying of raw material powders to provide Ex.O within a predetermined range, by carrying out the mechanical alloying in an Ar atmosphere having a super purity of not less than 99.9999%, by reducing stirring energy during the mechanical alloying or by using a metal Y powder or an Fe 2 Y powder in place of the Y 2 O 3  powder.

TECHNICAL FIELD

The present invention relates to an oxide dispersion strengthened (ODS)martensitic steel excellent in high-temperature strength and a method ofmanufacturing this steel.

The oxide dispersion strengthened martensitic steel of the presentinvention can be advantageously used as a fuel cladding tube material ofa fast breeder reactor, a first wall material of a nuclear fusionreactor, a material for thermal power generation, etc. in whichexcellent high-temperature strength and creep strength are required.

BACKGROUND ART

Although austenitic stainless steels have hitherto been used in thecomponent members of nuclear reactors, especially fast reactors whichare required to have excellent high-temperature strength and resistanceto neutron irradiation, they have limitations on irradiation resistancesuch as swelling resistance. On the other hand, martensitic stainlesssteels have the disadvantage of low high-temperature strength althoughthey are excellent in irradiation resistance.

Therefore, oxide dispersion strengthened martensitic steels have beendeveloped as materials that combine irradiation resistance andhigh-temperature strength and there have been proposed techniques forimproving high-temperature strength by adding Ti to oxide dispersionstrengthened martensitic steels, thereby finely dispersing oxideparticles.

For example, Japanese Patent Laid-Open No. 5-18897/1993 discloses atempered oxide dispersion strengthened martensitic steel whichcomprises, as expressed by % by weight, 0.05 to 0.25% C, not more than0.1% Si, not more than 0.1% Mn, 8 to 12% Cr (12% being excluded), 0.1 to4.0% in total of Mo+W, not more than 0.01%0 (O in Y₂O₃ and TiO₂ beingexcluded) with the balance being Fe and unavoidable impurities, and inwhich complex oxide particles comprising Y₂O₃ and TiO₂ having an averageparticle diameter of not more than 1000 A are homogeneously dispersed inthe matrix in an amount of 0.1 to 1.0% in total of Y₂O₃+TiO₂ and in therange of 0.5 to 2.0 of the molecular ratio TiO₂/Y₂O₃.

However, even when oxide dispersion strengthened martensitic steels areproduced by adjusting the total amount of Y₂O₃ and TiO₂ and the ratio ofthese oxides and besides the total amount of Mo and W as disclosed inthe Japanese Patent Laid-Open No. 5-18997/1993, there are cases whereoxide particles are not finely dispersed in a homogeneous manner and itfollows that in such cases the expected effect on an improvement inhigh-temperature strength cannot be achieved.

DISCLOSURE OF THE INVENTION

An object of the present invention is, therefore, to provide an oxidedispersion strengthened martensitic steel in which oxide particles arefinely and homogeneously dispersed at a high density is positivelyobtained, with the result that excellent high-temperature strength isobtained, and to provide a method of manufacturing this steel.

Paying attention to the fact that an excess oxygen content Ex.O (a valueobtained by subtracting an oxygen content in Y₂O₃ from an oxygen contentin steel) in an oxide dispersion strengthened martensitic steel has aclose relation to high-temperature strength, the present inventors havefound that high-temperature strength can be positively improved byadjusting the level of the excess oxygen content in steel within apredetermined range, thus having accomplished the present invention.

According to the present invention, there is provided an oxidedispersion strengthened martensitic steel excellent in high-temperaturestrength which comprises, as expressed by % by weight, 0.05 to 0.25% C,8.0 to 12.0% Cr, 0.1 to 4.0% W, 0.1 to 1.0% Ti, 0.1 to 0.5% Y₂O₃ withthe balance being Fe and unavoidable impurities and in which Y₂O₃particles are dispersed in the steel, characterized in that the oxideparticles are finely dispersed and highly densified by adjusting the Ticontent within the range of 0.1 to 1.0% so that an excess oxygen contentEx.O in the steel satisfies [0.22×Ti (% by weight)<Ex.O (% byweight)<0.46×Ti (% by weight)].

Incidentally, in the following descriptions of this specification, “%”denotes “% by weight” unless otherwise noted.

In the present invention, by adjusting the Ti content in steel withinthe range of 0.1 to 1.0% so that the excess oxygen content Ex.O in steelbecomes a predetermined range, it becomes possible to finely disperseoxide particles in steel and increase the density of them at a highlevel, with the result that it becomes possible to improve thehigh-temperature short-time strength and high-temperature long-timestrength of the steel.

The steel of the invention described above can be manufactured bysubjecting either element powders or alloy powders and a Y₂O₃ powder tomechanical alloying treatment in an Ar atmosphere. In this manufacturingprocess, by reducing the amount of oxygen which is included in thesteel, it is also possible to keep the excess oxygen content in theresulting steel in a predetermined range.

Accordingly, the present invention provides a method of manufacturing anoxide dispersion strengthened martensitic steel excellent inhigh-temperature strength, the method comprising subjecting eitherelement powders or alloy powders and a Y₂O₃ powder to mechanicalalloying treatment in an Ar atmosphere to manufacture an oxidedispersion strengthened martensitic steel which comprises 0.05 to 0.25%C, 8.0 to 12.0% Cr, 0.1 to 4.0% W, 0.1 to 1.0% Ti, 0.1 to 0.5% Y₂O₃ withthe balance being Fe and unavoidable impurities and in which Y₂O₃particles are dispersed in the steel, characterized in that an Ar gashaving a purity of not less than 99.9999% is used as the Ar atmosphereso that an excess oxygen content Ex.O in the steel satisfies [0.22×Ti (%by weight)<Ex.O (% by weight)<0.46×Ti (% by weight)].

The present invention further provides a method of manufacturing anoxide dispersion strengthened martensitic steel excellent inhigh-temperature strength, the method comprising subjecting eitherelement powders or alloy powders and a Y₂O₃ powder to mechanicalalloying treatment in an Ar atmosphere to manufacture an oxidedispersion strengthened martensitic steel which comprises 0.05 to 0.25%C, 8.0 to 12.0% Cr, 0.1 to 4.0% W, 0.1 to 1.0% Ti, 0.1 to 0.5% Y₂O₃ withthe balance being Fe and unavoidable impurities and in which Y₂O₃particles are dispersed in the steel, characterized in that a stirringenergy during the mechanical alloying treatment decreases to suppressoxygen contamination during stirring so that an excess oxygen contentEx.O in the steel satisfies [0.22×Ti (% by weight)<Ex.O (% byweight)<0.46×Ti (% by weight)].

The present invention further provides a method of manufacturing anoxide dispersion strengthened martensitic steel excellent inhigh-temperature strength, the method comprising subjecting eitherelement powders or alloy powders and a Y₂O₃ powder to mechanicalalloying treatment in an Ar atmosphere to manufacture an oxidedispersion strengthened martensitic steel which comprises 0.05 too.25%C, 8.0 to 12.0% Cr, 0.1 to 4.0% W, 0.1 to 1.0% Ti, 0.1 to 0.5% Y₂O₃ withthe balance being Fe and unavoidable impurities and in which Y₂O₃particles are dispersed in the steel, characterized in that a metal Ypowder or a Fe₂Y powder is used in place of the Y₂03 powder so that anexcess oxygen content Ex.O in the steel satisfies [0.22×Ti (% byweight)<Ex.O (% by weight)<0.46×Ti (% by weight)].

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the results of a creep rupture test at 700° C.of various test materials.

FIGS. 2A and 2B are graphs showing the results of a tensile test at 700°C. and 800° C. of the test materials MM11, T5 and MM13. The graph 2Ashows 0.2% proof stress and the graph 2B shows tensile strength.

FIG. 3 is transmission electron microphotographs of the test materialsMM11, T14, MM13 and T3 having an amount of added Ti of 0.2%.

FIG. 4 is transmission electron microphotographs of the test materialsT4 and T5 having an amount of added Ti of 0.5%.

FIG. 5 is a graph showing the relationship between the Ti content andthe excess oxygen content Ex.O of each test material. The diagonallyshaded portion indicates an area in which oxide particles can be finelydispersed and [Ex.O<0.46×Ti] is satisfied.

FIG. 6 is a graph showing the relationship between the measured valueand target value of excess oxygen content of each test material.

FIGS. 7A and 7B are graphs showing the results of a high-temperaturecreep rupture test at 700° C. of each test material. The graph 7A showsthe results of the creep rupture test and the graph 7B shows thedependence of rupture stresses at 1000 hours on the excess oxygencontent.

FIGS. 8A and 8B are graphs showing the dependence of the results of ahigh-temperature creep rupture test at 700° C. of each test material onTiOx (atomic percentage ratio of Ex.O/Ti) The graph 8A shows thedependence of estimated rupture stresses at 1000 hours on TiOx and thegraph 8B shows the dependence of tensile strength on TiOx.

FIG. 9 is a graph showing the relationship between the amount of Ticontent and excess oxygen content Ex.O of each test material.

BEST MODE FOR CARRYING OUT THE INVENTION

The chemical composition of the oxide dispersion strengthenedmartensitic steel of the present invention and the reasons for thelimitation of its components will be described below.

Cr (chromium) is an element important for ensuring corrosion resistance,and if the Cr content is less than 8.0%, the worsening of corrosionresistance becomes remarkable. If the Cr content exceeds 12.0%, adecrease in toughness and ductility is feared. For this reason, the Crcontent should be 8.0 to 12.0%.

When the Cr content is 8.0 to 12.0%, it is necessary that C (carbon) becontained in an amount of not less than 0.05% in order to make thestructure a stable martensite structure. This martensite structure isobtained by conducting heat treatment including normalizing at 1000 to1150° C.+tempering at 700 to 800° C. The higher the C content, theamount of precipitated carbides (M₂₃C₆, M₆C, etc.) and high-temperaturestrength increases. However, workability deteriorates if C is containedin an amount exceeding 0.25%. For this reason, the C content should be0.05 to 0.25%.

W (tungsten) is an important element which dissolves into an alloy in asolid solution state to improve high-temperature strength, and is addedin an amount of not less than 0.1%. A high W content improves creeprupture strength due to the solid solution strengthening, thestrengthening by carbide (M₂₃C₆, M₆C, etc.) precipitation and thestrengthening by intermetallic compound precipitation. However, if the Wcontent exceeds 4.0%, the amount of δ-ferrite increases and contrarilystrength decreases. For this reason, the W content should be 0.1 to4.0%.

Ti (titanium) plays an important role in the dispersion strengthening ofY₂O₃ and forms the complex oxide Y₂Ti₂O₇ or Y₂TiOs by reacting withY₂O₃, thereby functioning to finely disperse oxide particles. Thisaction tends to reach a level of saturation when the Ti content exceeds1.0%, and the finely dispersing action is small when the Ti content isless than 0.1%. For this reason, the Ti content should be 0.1 to 1.0%.

Y₂O₃ is an important additive which improves high-temperature strengthdue to dispersion strengthening. When the Y₂O₃ content is less than0.1%, the effect of dispersion strengthening is small and strength islow. On the other hand, when Y₂O₃ is contained in an amount exceeding0.5%, hardening occurs remarkably and a problem arises in workability.For this reason, the Y₂O₃ content should be 0.1 to 0.5%.

A method described below may be used as a general manufacturing methodof the oxide dispersion strengthened martensitic steel of the presentinvention. The above-described components as either element powders oralloy powders and a Y₂O₃ powder are mixed so as to obtain a targetcomposition. The resulting powder mixture is subjected to mechanicalalloying treatment which comprises charging the powder mixture into ahigh-energy attritor and stirring the powder mixture in an Aratmosphere. Thereafter, the resulting alloyed powder is filled in acapsule made of a mild steel. The capsule is then degassed and sealed,and hot extrusion is carried out after heating it to 1150° C. to therebysolidify the alloyed powder.

In this manufacturing process, an Ar gas having a purity of 99.99% isusually used as the atmosphere gas during the mechanical alloyingtreatment. However, even when such a high-purity Ar gas is used, it isimpossible to avoid the oxygen contamination into steel, though slightin quantity. In the present invention, by using a high purity Ar gas ofnot less than 99.9999%, it is possible to reduce the oxygencontamination into steel, with the result that it is possible to adjustthe excess oxygen content in the resulting steel within a predeterminedrange.

Furthermore, in carrying out the mechanical alloying treatment bycharging the raw material powder mixture into the high-energy attritorand stirring the powder mixture, by decreasing the stirring energy inthe attritor and suppressing the amount of entrapped oxygen during thestirring, it is also possible to reduce the excess oxygen content insteel and to adjust the excess oxygen content in the resulting steelwithin a predetermined range. As specific means of decreasing thestirring energy, it is considered to lower the rotary speed of anagitator of the attritor, to shorten the length of a pin attached to theagitator, and the like.

Moreover, in the step of mixing either element powders or alloy powdersand a Y₂O₃powder to prepare a target composition, a metal Y powder or anFe₂Y powder is used as a raw material powder in place of the Y₂O₃powder. By using such a metal Y powder or an Fe₂Y powder, the Y metalreacts with the oxygen which is contaminated during the manufacturingprocess such as the mechanical alloying treatment or with the oxygenfrom mixed unstable oxides (Fe₂O₃ etc.), to thereby formthermodynamically stable dispersed Y₂O₃ particles. As a result, it ispossible to effectively adjust the excess oxygen content in steel to apredetermined range. Incidentally, the excess oxygen content in steel inthis case is calculated on the assumption that the whole amount of theadded metal Y becomes Y₂O₃.

TEST EXAMPLE

Table 1 collectively shows the target compositions of test materials ofoxide dispersion strengthened martensitic steel, features of thecompositions, and manufacturing conditions. TABLE 1 Test material No.Target composition Features of compositions Manufacturing conditionsMM11 0.13C—9Cr—2W—0.20Ti—0.35Y₂O₃ Basic composition Stirring energy:Small Atmosphere: 99.99% Ar MM13 0.13C—9Cr—2W—0.20Ti—0.35Y₂O₃ Basiccomposition Stirring energy: Large Atmosphere: 99.99% Ar T140.13C—9Cr—2W—0.20Ti—0.35Y₂O₃ Basic composition Stirring energy: LargeAtmosphere: 99.99% Ar T3 0.13C—9Cr—2W—0.20Ti—0.35Y₂O₃—0.17Fe₂O₃ Additionof unstable Stirring energy: Large oxide (Fe₂O₃) Atmosphere: 99.99% ArT4 0.13C—9Cr—2W—0.50Ti—0.35Y₂O₃ Increase of Ti Stirring energy: LargeAtmosphere: 99.99% Ar T5 0.13C—9Cr—2W—0.50Ti—0.35Y₂O₃—0.33Fe₂O₃ Increaseof Ti Stirring energy: Large Addition of unstable Atmosphere: 99.99% Aroxide (Fe₂O₃) E5 0.13C—9Cr—2W—0.20Ti—0.35Y₂O₃ Basic composition Stirringenergy: Large Atmosphere: 99.9999% Ar

In each test material, either element powders or alloy powders and aY₂O₃ powder were blended to obtain a target composition, charged into ahigh-energy attritor and thereafter subjected to mechanical alloyingtreatment by stirring in an Ar atmosphere. The number of revolutions ofthe attritor was about 220 revolutions per minute (rpm) and the stirringtime was about 48 hours. The resulting alloyed powder was filled in acapsule made of a mild steel, degassed at a high temperature in avacuum, and then subjected to hot extrusion at about 1150 to 1200° C. inan extrusion ratio of 7 to 8:1, to thereby obtain a hot extrudedrod-shaped material.

In each of the test materials shown in Table 1, not only a Y₂O₃ powderbut also Ti was added to try to finely disperse and highly densifydispersed oxide particles by the formation of complex oxides of Ti andY. The test materials MM11, MM13, T14 and E5 have a basic composition.T3 is a test material in which the excess oxygen content wasintentionally increased by adding an unstable oxide (Fe₂O₃) to the basiccomposition of MM13 and T14. T4 is a test material in which the amountof added Ti was increased by adding higher amount of Ti powder to thebasic composition of M13 and T14. T5 is a test material in which theexcess oxygen content was increased by adding an unstable oxide (Fe₂O₃)and the amount of added Ti was also increased.

“Stirring energy” in the manufacturing conditions (mechanical alloyingtreatment conditions) of Table 1 shows the difference in the length ofthe pin attached to the agitator of the attritor which stirs the rawmaterial powders during the mechanical alloying treatment. “Stirringenergy: Large” means the use of the pin having a normal length, and“Stirring energy: Small” means the use of the pin having a lengthshorter than normal. That is, even when the number of revolutions of theagitator is the same, the stirring energy is smaller in the case of theshorter pin than in the case of the pin having a normal length and hencethe amount of entrapped oxygen is reduced during the stirring. For onlyMM11 in Table 1, an agitator which has the shorter pin and in which thestirring energy is small was used. In all other test materials, anagitator which has the pin of normal length and in which the stirringenergy is large was used. For the Ar atmosphere, a super high purity Argas having a purity of 99.9999% was used in only E5 in Table 1 and ahigh purity Ar gas having a purity of 99.99% was used in all other testmaterials.

Table 2 collectively shows the results of chemical analysis of each testmaterial which was prepared as described above. TABLE 2 Classifi-Chemical compositions (wt %) cation C Si Mn P S Ni Cr W Ti Y O N Ar Y₂O₃Ex. O Target 0.11˜0.15 <0.20 <0.20 <0.02 <0.02 <0.20 8.5˜9.5 1.8˜2.20.18˜0.22 0.26˜0.29 0.15˜0.25 <0.07 <0.007 range of basic compo- sitionTarget 0.13 — — — — — 9.00 2.00 0.20 0.275 0.20 — — value MM11 0.14<0.01 <0.01 0.002 0.003 <0.01 9.00 1.92 0.20 0.28 0.15 0.009 0.003 0.360.07 MM13 0.14 <0.005 <0.01 0.001 0.003 0.01 8.80 1.95 0.21 0.27 0.210.009 0.005 0.343 0.137 T14 0.14 <0.005 <0.01 0.002 0.003 0.04 8.80 1.960.21 0.26 0.18 0.013 0.005 0.330 0.110 T3 0.13 <0.005 <0.01 0.002 0.0030.01 8.75 1.93 0.21 0.27 0.22 0.012 0.005 0.343 0.147 T4 0.13 <0.005<0.01 0.002 0.003 0.01 8.72 1.93 0.46 0.27 0.18 0.009 0.005 0.343 0.107T5 0.13 <0.005 <0.01 0.002 0.003 0.01 8.75 1.93 0.46 0.27 0.24 0.0110.005 0.343 0.167 E5 0.13 <0.005 <0.01 <0.005 0.002 0.01 8.89 1.97 0.210.28 0.16 0.009 0.005 0.356 0.084

Creep Rupture Test

Among the hot extruded rod-shaped materials obtained above, T14, T3, T4,T5 and E5 were subjected to final heat treatment involving normalizing(1050° C.×1 hr, air cooling)+tempering (800° C.×1 hr, air cooling) andfinished as rod-shaped materials. MM11 and MM13 were first formed intubular shape and then subjected to final heat treatment involvingnormalizing (1050° C.×1 hr, air cooling)+tempering (800° C.×1 hr, aircooling). The tube making process was carried out by the first coldrolling+heat treatment for softening the second cold rolling+heattreatment for softening→the third cold rolling+heat treatment forsoftening→the fourth cold rolling+final heat treatment.

For rod-shaped test pieces (T14, T3, T4, T5, E5) and tubular test pieces(MM11, MM13) thus obtained, a creep rupture test at 700° C. wasconducted. The results of the test are shown in the graph shown inFIG. 1. For the rod like test pieces (T14, T3, T4, T5, E5), a gaugeportion of 6 mm diameter×30 mm length was worked for the test. From thisgraph, it is understood that the creep rupture strength of each of thetest materials MM11, T4, T5 and E5 is superior to that of other testmaterials. Since an oxide dispersion strengthened martensitic steel hasan equiaxed grain structure and does not have anisotropy in strength, acomparison between tubular test pieces and rod like test pieces ispossible.

Incidentally, the arrow in the graph shown in FIG. 1 indicates that arupture did not occur after a lapse of the test time and that the timeto rupture can be longer than shown in the figure.

Tensile Strength Test

For the test materials MM13, MM11 and T5, a tensile strength test wasconducted at test temperatures of 700° C. and 800° C. The results of thetest are shown in the graphs shown in FIGS. 2A and 2B. For MM11 andMM13, tubular test pieces similar to those used in the creep rupturetest were used. Because hoop strength is important when test materialsare used as materials for tubes, a gauge portion was provided in thehoop direction of a tubular test piece of 6.9 mm diameter×0.4 mm wallthickness (MM13) or of 8.5 mm diameter×0.5 mm wall thickness (MM11) anda hoop tensile strength test (a ring tensile test) was conducted. Thelength of the gauge portion was 2 mm and the width thereof was 1.5 mm.In T5, which is a rod-shaped material, a gauge portion of 6 mmdiameter×30 mm length was provided and an axial tensile strength testwas conducted. Since an oxide dispersion strengthened martensitic steelhas an equiaxed grain micro-structure and almost does not haveanisotropy in strength, it is possible to make a comparison between theresults of the tensile strength test of MM13 and MM11 and the results ofthe tensile strength test of T5. In accordance with JIS Z2241, thestrain rate was set at 0.1%/min to 0.7%/min.

As is understood from the graphs shown in FIGS. 2A and 2B, the testmaterials MM11 and T5 are superior to the test material MM13 of thebasic composition in both 0.2% proof stress and tensile strength.

Microscopic Observation

For each of the test materials prepared by subjecting the hot extrudedrod-shaped materials obtained above to heat treatment for normalizing(1050° C.×1 hr), an observation by a transmission electron microscope(TEM) was carried out. The results of the microscopic observation areshown in FIG. 3 (test materials having an amount of added Ti of 0.2%)and in FIG. 4 (test materials having an amount of added Ti of 0.5%).

In FIG. 3, the test material MM11 shows Y₂O₃ particles which are morefinely dispersed and more increased in density at a higher level thanT14, MM13 and T3. In FIG. 4, both T4 and T5 show Y₂O₃ particles whichare finely dispersed and increased in density.

Ti Content and Excess Oxygen Content

For each of the test materials, the relationship between the Ti contentand the excess oxygen content (Ex.O) shown in the results of chemicalanalysis in Table 2 are illustrated in the graph shown in FIG. 5. Eachof the test materials MM11, T4, T5 and E5 included in the diagonallyshaded portion of this graph is excellent in creep rupture strength andtensile strength and shows Y₂O₃ particles which are finely dispersed andhighly densified. Namely, it is understood that at Ti contents of notless than 0.1%, test materials which satisfy the relationship of excessoxygen content (Ex.O)<0.46×Ti produce oxide dispersion strengthenedmartensitic steels in which Y₂O₃ particles are finely dispersed andhighly densified and which are excellent in high-temperature strength.

Incidentally, in the graph shown FIG. 5, a lower limit of the excessoxygen content Ex.O expressed by [0.22×Ti (% by weight)<Ex.O (% byweight)] is not examined. The lower limit will be described referring toFIGS. 8 and 9, which will be described later.

Adjustment of Ti Content

A comparison between the test material MM13 of basic composition (Ticontent: 0.21%, excess oxygen content 0.137>0.46×Ti) and the testmaterial T4 in which the Ti content was increased (Ti content: 0.46%,excess oxygen content 0.107<0.46×Ti) reveals that T4 shows dispersedY₂O₃ particles which are more finely dispersed and more increased indensity at a higher level and has higher creep rupture strength.

In the test material T3 (Ti content: 0.21%, excess oxygen content0.147>0.46×Ti) in which the excess oxygen content was intentionallyincreased by adding Fe₂O₃ to the test material MM13.of the basiccomposition, dispersed Y₂O₃ particles are more coarsened than the testmaterial MM13 of the basic composition and creep rupture strength alsodecreases. However, by adding a further increased amount of Ti to thetest material T3 in which the excess oxygen content was increased, it ispossible to make the excess oxygen content less than 0.46×Ti % as seenin the test material T5 (Ti content: 0.46%, excess oxygen content0.167<0.46×Ti), to more finely disperse and more highly densifydispersed Y₂O₃ particles at a higher level than T3, and to improve thecreep rupture strength.

From these facts, it is understood that in the oxide dispersionstrengthened martensitic steel in which the Ti content in steel isadjusted within the range of 0.1 to 0.5% so that the excess oxygencontent becomes less than 0.46×Ti, Y₂O₃ particles are finely dispersedand highly densified and the high-temperature strength of this steel isexcellent.

Purity of Ar Gas

Even in the test material E5 (excess oxygen content 0.084<0.46×Ti)having the same composition as the test material MM13 of the basiccomposition (excess oxygen content 0.137>0.46×Ti), by changing thepurity of Ar gas used in the Ar atmosphere during mechanical alloyingtreatment from a high purity of 99.99% to a super high purity of99.9999%, it is possible to reduce the oxygen contamination during thestirring in the attritor and hence the excess oxygen content in steelcan be held to less than 0.46×Ti %.

From this fact, it is understood that by using a super high purity Argas of not less than 99.9999% as the Ar atmosphere during mechanicalalloying treatment, it is possible to obtain an oxide dispersionstrengthened martensitic steel in which Y₂O₃ particles are finelydispersed and highly densified and which is excellent inhigh-temperature strength.

Adjustment of Stirring Energy During Mechanical Alloying Treatment

A comparison between the test material MM13 of the basic composition(excess oxygen content 0.137>0.46×Ti) and the test material MM11 of thesame composition (excess oxygen content 0.07<0.46×Ti) reveals that inthe test material MM11 which was obtained by reducing stirring energyduring mechanical alloying treatment by use of a pin attached to theagitator in the attritor having a length shorter than normal length, itis possible to hold the excess oxygen content to less than 0.46×Ti %.

In the test material MM11, Y₂O₃ particles can be finely dispersed andhighly densified in comparison with the test material MM13 and creeprupture strength and tensile temperature strength can be improved.

From this fact, it is understood that by reducing the stirring energyduring mechanical alloying treatment to limit the amount of entrappedoxygen during stirring, it is possible to obtain an oxide dispersionstrengthened martensitic steel in which Y₂O₃ particles are finelydispersed and highly densified and which is excellent inhigh-temperature strength.

Use of Metal Y Powder in Place of Y₂O₃ Powder

Table 3 collectively shows the target compositions and the target excessoxygen contents of the test materials. Incidentally, E5 and T3 in Table3 are the same as the test materials in Table 1.

E5 and E7 are standard materials of the basic composition to which aY₂O₃ powder is added and the target excess oxygen content is 0.08%. Y1,Y2 and Y3 are materials to which a metal Y powder is added in place of aY₂O₃ powder. That is, in Y1, a metal Y powder is added without theaddition of an unstable oxide (Fe₂O₃) and the target excess oxygencontent is 0%. In Y2 and Y3, a Fe₂O₃ powder, along with a metal Ypowder, is added in an amount of 0.15% and 0.29%, respectively, and thetarget excess oxygen content is 0.05% and 0.09%, respectively. In T3,the excess oxygen content is increased by adding Fe₂O₃ powder to thebasic composition of E5 and E7.

The test materials Y1, Y2, Y3 and E7 were all produced as hot extrudedrod-shaped materials by the same manufacturing method and under the samemanufacturing conditions as with MM13 described above, and heating andcooling in furnace (1050° C.×1 hr→600° C. (30° C./hr)) or normalizing(1050° C.×1 hr·air cooling)+tempering (780° C.×1 hr·air cooling) wascarried out as final heat treatment.

The results of chemical analysis of each test material are collectivelyshown in Table 4. TABLE 3 Test material Target composition Feature Y10.13C—9Cr—2W—0.2Ti—0.28Y Target excess oxygen content: 0 wt % Y20.13C—9Cr—2W—0.2Ti—0.28Y—0.15Fe₂O₃ Target excess oxygen content: 0.05 wt% Y3 0.13C—9Cr—2W—0.2Ti—0.28Y—0.29Fe₂O₃ Target excess oxygen content:0.09 wt % E5, E7 0.13C—9Cr—2W—0.20Ti—0.35Y₂O₃ Standard material (targetexcess oxygen content: 0.08 wt %) T30.13C—9Cr—2W—0.20Ti—0.35Y₂O₃—0.17Fe₂O₃ Excess oxygen added-material(target excess oxygen content: 0.13 wt %)

TABLE 4 Chemical compositions (wt %) C Si Mn P S Ni Cr W Ti Y O N ArY₂O₃ Ex. O Y1 0.13 0.012 <0.01 <0.005 0.002 0.01 8.85 1.93 0.20 0.270.099 0.014 0.0054 0.34 0.026 Y2 0.13 0.005 <0.01 <0.005 0.002 0.01 8.871.96 0.21 0.28 0.12 0.012 0.0055 0.36 0.044 Y3 0.14 0.020 <0.01 <0.0050.002 <0.01 8.86 1.97 0.21 0.28 0.18 0.010 0.0050 0.36 0.104 E7 0.140.007 0.02 <0.005 0.003 0.02 8.92 1.97 0.20 0.27 0.16 0.0099 0.0047 0.340.087 E5 0.13 <0.005 <0.01 <0.005 0.002 0.01 8.89 1.97 0.21 0.28 0.160.0087 0.0048 0.36 0.084 T3 0.13 <0.005 <0.01 0.002 0.003 0.01 8.75 1.930.21 0.27 0.22 0.012 0.0049 0.34 0.147

FIG. 6 is a graph showing the relationship between the measured valueand target value of excess oxygen content of each test material. Thetarget oxygen content was set taking into consideration the oxygencontamination of about 0.04% from the raw material powders and about0.04% during mechanical alloying treatment, that is, 0.08% in total, inaddition to oxygen brought from the Fe₂O₃ power and Y₂O₃ powder.Incidentally, the impurity oxygen content in the raw material powders(Fe, Cr, W, Ti) and the content of oxygen inclusion during mechanicalalloying treatment were determined by measuring the chemicalcompositions in the raw material powders and in alloys after mechanicalalloying treatment, respectively, by an inert gas fusion method.

From FIG. 6, it is understood that even at low content of excess oxygenof not more than 0.1%, agreement is almost obtained between the targetvalues and measured values of excess oxygen content and that Y₂O₃ isformed by the combined addition of metal Y and Fe₂O₃, with the resultthat the excess oxygen content can be controlled in a low range of notmore than 0.1%.

FIGS. 7A and 7B show the results of high-temperature creep test for eachtest material at 700° C. FIG. 7A is a graph showing the results of thecreep rupture test and FIG. 7B is a graph showing the dependence ofrupture stresses at 1000 hours on the excess oxygen content. In the testmaterials E5 and E7 having the excess oxygen content of about 0.08%, thehigh-temperature creep strength reaches a peak, and the strength tendsto decrease at before and after 0.08%. From this fact, it is understoodthat the adjustment of the excess oxygen content at low levels of about0.08% is effective in improving high-temperature strength and that it iseffective to add a metal Y powder in place of a Y₂O₃ powder as controlmeans of the excess oxygen content at such low levels. It is furtherunderstood that, since excessive lowering of the excess oxygen contentresults in a decrease in high-temperature strength, it is necessary toset not only an upper limit of the excess oxygen content, which is lessthan 0.46×Ti %, but also a lower limit of the excess oxygen content insteel.

FIGS. 8A and 8B show the dependence of the results of a high-temperaturecreep test at 700° C. of each test material on TiOx (atomic percentageratio of Ex.O/Ti). FIG. 8A is a graph showing the dependence ofestimated rupture stresses at 1000 hours on TiOx and FIG. 8B is a graphshowing the dependence of tensile strength on TiOx. From these graphs,it is understood that the creep strength and tensile strength reach apeak in the TiOx range of 0.65 to 1.4 (diagonally shaded portion).

FIG. 9 is a graph showing the relationship between the amount of addedTi and excess oxygen content Ex.O of each test material, and the rangeshowing the peak of creep strength in FIG. 8, namely [0.65×Ti (atomic%)<Ex.O (atomic %)<1.4×Ti (atomic %)], is indicated by oblique lines.When the above-described relationship expressed by atomic % is convertedto % by weight, there can be described as follows: [0.22×Ti (% byweight)<Ex.O (% by weight)<0.464×Ti (% by weight)].

As described above, Ti forms complex oxides by reacting with a Y₂O₃powder, thereby functioning to finely disperse oxide particles. Thisaction tends to reach a level of saturation when the Ti content exceeds1.0%, and becomes small when the Ti content is less than 0.1%. From thisfact, when the amount of added Ti is in the range of 0.1% to 1.0%, bycontrolling the excess oxygen content within the range of [0.22×Ti (% byweight)<Ex.0 (% by weight)<0.464×Ti (% by weight)], namely, within thediagonally shaded range in the graph of FIG. 9, it is possible tomanufacture an oxide dispersion strengthened martensitic steel excellentin high-temperature strength.

Industrial Applicability

As is apparent from the above descriptions, according to the presentinvention, by paying attention to the excess oxygen content in steel, itis possible to positively obtain a structure in which oxide particlesare finely dispersed and highly densified by adjusting the Ti content orby reducing the amount of oxygen contamination during the manufacturingprocess so that the excess oxygen content becomes within a predeterminedrange. As a result, it is possible to provide an oxide dispersionstrengthened martensitic steel excellent in high-temperature strength.

1. An oxide dispersion strengthened martensitic steel excellent inhigh-temperature strength which comprises, as expressed by % by weight,0.05 to 0.25% C, 8.0 to 12.0% Cr, 0.1 to 4.0% W, 0.1 to 1.0% Ti, 0.1 to0.5% Y₂O₃ with the balance being Fe and unavoidable impurities and inwhich Y₂O₃ particles are dispersed in the steel, characterized in thatthe oxide particles are finely dispersed and highly densified byadjusting the Ti content within the range of 0.1 to 1.0% so that anexcess oxygen content Ex.0 in the steel satisfies [0.22×Ti (% byweight)<Ex.O (% by weight)<0.46×Ti (% by weight)], the excess oxygencontent Ex.O being a value obtained by subtracting an oxygen content inY₂O₃ from an oxygen content in the steel.
 2. A method of manufacturingan oxide dispersion strengthened martensitic steel excellent inhigh-temperature strength, said method comprising subjecting eitherelement powders or alloy powders and a Y₂O₃ powder to mechanicalalloying treatment in an Ar atmosphere to manufacture an oxidedispersion strengthened martensitic steel which comprises, as expressedby % by weight, 0.05 to 0.25% C, 8.0 to 12.0% Cr, 0.1 to 4.0% W, 0.1 to1.0% Ti, 0.1 to 0.5% Y₂O₃ with the balance being Fe and unavoidableimpurities and in which Y₂O₃ particles are dispersed in the steel,characterized in that an Ar gas having a purity of not less than99.9999% is used as the Ar atmosphere so that an excess oxygen contentEx.O in the steel satisfies [0.22×Ti (% by weight)<Ex.O (% byweight)<0.46×Ti (% by weight)], the excess oxygen content Ex.O being avalue obtained by subtracting an oxygen content in Y₂O₃ from an oxygencontent in the steel.
 3. A method of manufacturing an oxide dispersionstrengthened martensitic steel excellent in high-temperature strength,said method comprising subjecting either element powders or alloypowders and a Y₂O₃ powder to mechanical alloying treatment in an Aratmosphere to manufacture an oxide dispersion strengthened martensiticsteel which comprises, as expressed by % by weight, 0.05 to 0.25% C, 8.0to 12.0% Cr, 0.1 to 4.0% W, 0.1 to 1.0% Ti, 0.1 to 0.5% Y₂O₃ with thebalance being Fe and unavoidable impurities and in which Y₂O₃ particlesare dispersed in the steel, characterized in that a stirring energyduring the mechanical alloying treatment decreases to suppress oxygencontamination during stirring so that an excess oxygen content Ex.O inthe steel satisfies [0.22×Ti (% by weight)<Ex.O (% by weight)<0.46×Ti (%by weight)], the excess oxygen content Ex.O being a value obtained bysubtracting an oxygen content in Y₂O₃ from an oxygen content in thesteel.
 4. A method of manufacturing an oxide dispersion strengthenedmartensitic steel excellent in high-temperature strength, said methodcomprising subjecting either element powders or alloy powders and a Y₂O₃powder to mechanical alloying treatment in an Ar atmosphere tomanufacture an oxide dispersion strengthened martensitic steel whichcomprises, as expressed by % by weight, 0.05 to 0.25% C, 8.0 to 12.0%Cr, 0.1 to 4.0% W, 0.1 to 1.0% Ti, 0.1 to 0.5% Y₂O₃ with the balancebeing Fe and unavoidable impurities and in which Y₂O₃ particles aredispersed in the steel, characterized in that a metal Y powder or a Fe₂Ypowder is used in place of the Y₂O₃ powder so that an excess oxygencontent Ex.0 in the steel satisfies [0.22×Ti (% by weight)<Ex.0 (% byweight)<0.46×Ti (% by weight)], the excess oxygen content Ex.0 being avalue obtained by subtracting an oxygen content in Y₂O₃ from an oxygencontent in the steel.